Isolation valves are designed to provide a double isolation barrier; typically, this is achieved either with double piston effect seats (ball or slab gate) valves or with double expanding gate valves. Both of these valve types are non-self-relieving. In fact, each seat is designed to provide a tight seal regardless of whether the pressure is acting from line-to-body or from body-to-line. This means that an eventual overpressure is retained in the body cavity by the specific seat design.
When double barrier isolation valves are exposed to thermal sources (such as, for example, radiation, proximity to hot equipment, and the like), the rise of valve body temperature can lead to an unacceptable increase in pressure.
The present pressure relief device maintains the valve body cavity pressure within acceptable limits.
An important issue in pressure relief device design is where the excess of pressure should be directed. Assuming that release to the atmosphere is not practicable, the aim is to convey the excess fluid from the cavity to the high pressure (HP) side of the valve. In fact, the low pressure side should be isolated by the shut-off valve, which when closed has the function of preventing fluid from entering the low pressure conduit.
In general, however, such isolation valves are bi-directional. In other words, either valve end can be the HP side when the valve is closed. What is needed, is a pressure relief device that can discern or “understand” which is the actual high pressure side and convey the body overpressure toward the HP side.